1. Field of the Invention
This invention pertains generally to sampling airborne particulate matter, and more particularly to a universal inlet device for size-selective sampling of airborne particles which can be adjusted for different particle cut sizes and different sampling flow rates.
2. Description of the Background Art
Inhalable airborne particles are commonly sampled using size-selective air sampling inlets. Such inlets are generally designed to remove particles which are larger than a prescribed aerodynamic size, so that the particles collected by a downstream collector are limited in size to those which are inhalable by humans. Particles that pass through the inlet are typically collected on filters or by other means for analysis. The samples are then evaluated to determine possible heath risks from inhaling the particles.
For PM-10 sampling, government regulations require the air sampling instrument to be preceded by a sampling inlet that collects particles that tend to be larger than 10 micrometer in aerodynamic equivalent diameter following a prescribed efficiency standard (Title 40, Part 53 of the Code of Federal Regulations). A 50% collection efficiency is achieved for particles of aerodynamic diameter of 10.+-.0.5 .mu.m. The smaller particles can then pass to the subsequent collector, usually an efficient filter. A tolerance of 10% is applied to the total collected sample. The particles collected on the filter are retained as the PM-10 sample, and larger particles collected by the inlet are discarded when the inlet is cleaned.
Similar size-selective samplers are also used in industry for occupational and industrial hygiene and safety purposes. However, other cut sizes may be appropriate in those applications, such as the use of a cut size of 3.5 micrometer aerodynamic diameter for collecting respirable dust samples that describe particles that can penetrate to the deep lung if inhaled by people. In each of those cases, a tolerance of about 10% to 20% is usually applied to the ideal cut size to allow for normal variability in inlet performance.
Although there have been some specific sampling inlets designed previously, especially for the collection of PM-10 samples at about 1200 LPM, these inlets have been of relatively complex designs and have been flow-rate specific. That is, each of the existing designs can only be used to sample at one specified volumetric flow rate of air. If the PM-10 application requires a different flow rate or a different cut size is needed, existing designs are not directly usable.
Therefore, there is a need for a size-selective sampling inlet which can be scaled to meet different flow rates and cut size requirements. The present invention satisfies those needs, as well as others, and overcomes the deficiencies found in existing inlet designs.